The purpose of the proposed research is to gain an understanding of certain characteristics, genetic and phylogenetic, of the Cyanophyta or blue-green bacteria. Using unicellular species of the genera Anacystis and Synechococcus, various auxotrophic and temperature-sensitive strains have been isolated and are being biochemically characterized where possible. These mutants are employed in analyses of mechanisms of biosynthetic control, primarily repression, and in studies characterizing various modes of uptake of amino acids by growing cells. It is hoped that these investigations will allow improved methods for mutant isolation by indicating the reasons for present difficulties in such endeavors.